The present invention relates to cylindrical wall magnetic domain memories and, more specifically, to a bonded bubble memory device and to a method of producing the device.
Magnetic bubble domains constitute a type of domain characteristic of thin film magnetic materials which possess a uniaxial magnetic anisotropy, the easy direction of magnetization being perpendicular to the plane of the film. In a typical bubble memory device, a solid solution of a ferrimagnetic iron garnet is epitaxially grown on a substrate made of a single crystal, and a non-magnetic garnet material. To create the bubbles in the magnetic thin film, a DC magnetic field of a predetermined value is applied perpendicular to the plane of the film. The bubbles are mobile within the magnetic thin film, and the mobility is controlled through the combined effect of an in-plane, rotating magnetic field and a desired pattern of soft magnetic elements located near the plane of the film. These magnetic elements are often combined with conductive elements to form various known circuits, such as the domain detector, transfer switch, generator and annihilator.
In the conventional bubble memory device and method of fabricating the device, the pattern of magnetic elements and conductive elements is deposited on the domain sustaining film. Usually, however, an appropriate oxide coating is first formed on this film to provide a spacer layer between the film and the pattern of elements. The result is a multilayer thin film structure on a single substrate, the structure comprising the bubble film, the oxide coating, the magnetic elements and the conductive elements.
There are several inherent problems with a bubble memory device fabricated as just described. A multilayer thin film structure has a low yield, since a defect generated in any particular layer, usually prohibits the salvaging of the preceeding layers. For example, in the formation of the oxide layer, the garnet crystal is usually heated to about 450.degree. C., and this heat may cause annealing in the crystal, magnetic thin film layer which in turn may create a shift in the magnetic properties of the layer. Furthermore, the pattern magnetic elements and conductive elements can not be prescreened for defects and performance.